Audio matrix encoding and decoding is well known in the prior art. For example, in so-called “4-2-4” audio matrix encoding and decoding, four source signals, typically associated with the same four cardinal input and output directions (such as, for example, left, center, right and surround or left front, right front, left back and right back) are amplitude-phase matrix encoded into two signals in which their relative amplitude and polarity represents their directional encoding. The two signals are transmitted or stored and then decoded by an amplitude-phase matrix decoder in order to recover approximations of the original four source signals. The decoded signals are approximations because matrix decoders suffer the well-known disadvantage of crosstalk among the decoded audio signals. Ideally, the decoded signals should be identical to the source signals, with infinite separation among the signals. However, the inherent crosstalk in matrix decoders typically results in only 3 dB separation between signals associated with adjacent directions. An audio matrix in which the matrix characteristics do not vary is known in the art as a “passive” matrix. The quiescent or “unsteered” condition of an active or adaptive matrix is also referred to as its “passive” matrix condition.
In order to overcome the problem of crosstalk in matrix decoders, it is known in the prior art to vary adaptively the decoding matrix characteristics in order to improve separation among the decoded signals and more closely approximate the source signals. One well known example of such an active matrix decoder is the Dolby Pro Logic decoder, described in U.S. Pat. No. 4,799,260, which patent is incorporated by reference herein in its entirety. The '260 patent cites a number of patents that are prior art to it, many of them describing various other types of adaptive matrix decoders.
Improved types of adaptive matrix decoders are disclosed in U.S. patent application Ser. No. 09/454,810 of James W. Fosgate, filed Dec. 3, 1999 and U.S. patent application Ser. No. 09/532,711 of James W. Fosgate, filed Mar. 22, 2000 (the “Fosgate applications”). In said Fosgate applications, desirable relationships among intermediate signals in adaptive matrix decoders are used to simplify the decoder and improve the decoder's accuracy.
In the decoders of the Fosgate applications, Lt and Rt (“left total” and “right total”) input signals are received and four output signals are provided, the four output signals representing principal directions, left, right, center and surround, in which pairs of the directions (left/right, center/surround) lie on directions that are ninety degrees to each other. The relative magnitude and polarity of the Lt and Rt input signals carry directional information. A first “servo” operates on Lt and Rt and a second “servo” operates on the sum and difference of Lt and Rt, each servo delivering a pair of intermediate signals. The pair of intermediate signals delivered by each servo are controlled in magnitude and the controlled intermediate signals are “urged towards equality” or “controlled for equal magnitude” (but their polarities need not be the same) by the respective servo (hence, the appellation “servo”). The four decoder output signals are generated by combining, both additively and subtractively, each pair of magnitude controlled “urged toward equality” intermediate signals.
The four-output decoders disclosed in said Fosgate applications are “perfect” in the sense that a single source signal having a particular direction encoded into the input Lt and Rt signals is reproduced (with appropriate relative magnitudes) only by the two outputs representing directions adjacent to the encoded direction (or, when the encoded direction happens to be exactly the direction represented by an output, only by that single output).
The second of said Fosgate applications also discloses a technique for providing decoder outputs for directions other than the directions of the four outputs derived from pairs of intermediate signals controlled for equal magnitude. However, such additional decoder output signals suffer from greater undesirable crosstalk than the basic four outputs of the decoders in said Fosgate applications. Thus, despite the improved performance provided by the decoders in said Fosgate applications, there remains a need for an adaptive matrix decoder capable of providing multiple outputs, each having an arbitrary direction, the outputs having the high degree of crosstalk suppression of the four-output decoders of said Fosgate applications.